DETERMINATION OF L-ASCORBIC-ACID IN FRUIT AND VEGETABLE JUICES BY FLOW-INJECTION WITH IMMOBILIZED ASCORBATE OXIDASE

Ascorbate oxidase was immobilised on cyanogen bromide activated-Sepharose 4B and incorporated in a flow-injection system with amperometric detection at a glassy carbon electrode at +0.6 V. On passage through the immobilised ascorbate oxidase a fraction of the L-ascorbic acid was converted into dehydroascorbic acid and the decrease in signal was measured. This could be directly related to the amount of L-ascorbic acid present. The calibration graph was linear over the range 0-400 ng ml-1 with a correlation coefficient of 0.9994. The detection limit (2σ) in phosphate buffer (0.08 M, pH 5.5) was 4.0 ng ml-1. The relative standard deviation for a 200 ng ml-1 standard was 1.0% (n = 10) and the sampling throughput was 30 samples h -1. The method was used for the simple and rapid determination of L-ascorbic acid in fruit and vegetable juice.